1. Field of the Invention
The present invention relates to an apparatus for positioning and clamping a work holding pallet on a work table of a machine tool.
When a large number of relatively small work pieces are arranged on a work table of a numerically controlled (NC) machine tool for machining, substantial effort and care are required to precisely position them and subsequently clamped them to the work table.
Although great improvement of the machine's performance has been achieved, much less improvement has been shown in automating the work-changing steps.
Hence, despite the high machining performance, the net working ratio is not more than 30.about.40% even for the latest models of automated machine tools.
The significance of improving this area is now being recognized and work-changing systems using work-clamp pallets are now being put to practical use. These systems are designed to increase the speed and the efficiency of the work-changing operation so as to sizably improve the overall working efficiency and productivity of the machine tools through reduction of the down-time required to change the workpieces.
The basic idea of these systems is to position and clamp the workpieces on a work-clamp pallet outside the work table of the machine tool, and transfer the work-clamp pallet (with a large number of workpieces on it) onto the work table in exchange for the finished one, i.e. a work-clamp pallet with finished workpieces to thus change the workpieces quickly and efficiently.
The present invention relates to an apparatus for positioning and clamping such workpiece holding pallets on a work table of the machine tool.
2. Description of the Prior Art
One of the latest prior art disclosures concerning an apparatus for positioning and clamping a work-clamp pallet for machine tools is U.S. Pat. No. 4,179,106 titled "Apparatus for Locating and Clamping a Work Member Supporting Fixture".
In the locating and clamping apparatus 110 of the prior art (shown in FIGS. 28 and 29) an auxiliary table 101 formed as the work table of a machine tool itself or fixedly secured to the work table 100 is provided with a transfer means 102 of the air film type, a centering device 103, a plurality of positioning devices 104 and clamping devices (not shown). A work member supporting fixture 105 is transferred onto this auxiliary table 101 to be located and clamped thereon.
The abovementioned transfer 102 consists of a large number of compressed air outlets in the topside of the auxiliary table 101, each provided with a valve ball for opening/closing thereof, which are opened when the work member supporting fixture 105 is placed thereon to form an air film under the supporting fixture.
The locating means comprises a centering device 103 (consisting of a pin projecting upwardly from the auxiliary table 101), and a plurality of the positioning devices 104 each consisting of a hydraulic cylinder extendable from the surface. The clamping devices are hydraulic clamping devices carried in T-sectioned grooves 106 with their positions freely adjustable along the edges of the work member supporting fixture 105.
The locating and clamping apparatus 110 described above is primarily for large-sized work member supporting fixtures 105. More particularly, this apparatus is used for work member supporting fixtures 105 mainly for the horizontal type machine tools having a low work-exchange frequency and is known to have the following defects or shortcomings:
(1) As is apparent from the fact that its transfer device 102 is of the air film type, the topside 107 of the auxiliary table 101 is used for transferring the supporting fixture 105 thereon while at the same time it is the plane on which the same is located and clamped, hence there is an increased risk of chips being caught between the topside of the auxiliary table 101 and the bottom of the work member supporting fixture 105. This may result in the work member supporting fixture 105 being improperly clamped, increasing the work positioning error with respect to the table 100;
(2) The pin of the centering device 103 projects above the topside of the auxiliary table 101, thus interfering with sideway transfer of the work member supporting fixture 105 onto the auxiliary table 101.
Moreover, while the centering pin is provided for centering of the work member suporting fixture 105, the fixture 105 is free to turn when it floats on the air film, this interfering with precise and efficient positioning thereof;
(3) The clamping devices are made to clamp the work member supporting fixture 105 along its edges, hence the T-sectioned grooves 106 in the topside of the auxiliary table 101 are indispensable. This requires the auxiliary table 101 to be thick and heavy enough to allow for the depth of the T-sectioned grooves 106. This is a fatal defect for the movable table 100 of an NC machine tool or the like which is required to be quickly shiftable with a high precision; and,
(4) The transfer device 102 of the air film type is high in compressed air consumption, hence increasing its energy cost.
The consumption of compressed air is bound to be increased remarkably especially when this type of transfer device 102 is applied to the auxiliary table 106 with the T-sectioned grooves in the topside thereof.